1. Field of the Invention
This invention relates to a charging member, a process cartridge and an image forming apparatus. More particularly, it relates to a charging member which is, in image forming apparatus such as copying machines and page printers, to be brought into contact with an electrophotographic photosensitive member to charge it electrostatically; and a process cartridge and an image forming apparatus which have such a charging member.
2. Related Background Art
As a charging means for primarily charging the electrophotographic photosensitive member serving as an image bearing member, corona charging assemblies, by which a high voltage is applied to a wire to cause corona discharge to take place and the photosensitive member, which is a member to be charged, is exposed to the corona thus produced, have conventionally been in wide use in electrophotographic image forming apparatus.
In recent years, a contact charging type charging means has also been developed and put into practical use, which is a charging means in which a charging member is so disposed as to be brought into contact with the photosensitive member and a voltage is applied to the charging member to charge the surface of the photosensitive member electrostatically.
This contact charging is in wide use because of its advantages that the applied voltage necessary for the photosensitive member to have the desired potential on its surface to be charged may be lower than that of the non-contact type corona charging and may generate ozone in a small quantity at the time of charging.
An example of a charging member in a conventional electrophotographic image forming apparatus is shown in FIG. 6.
A charging roller 2', which is the charging member, is constituted of a conductive support 21' serving also as a feeder electrode, an elastic layer 22' provided thereon, a resistance layer 23' further provided thereon and a protective layer 24' still further provided thereon.
The elastic layer 22' is formed of a synthetic rubber, where a solid rubber such as styrene-butadiene rubber (SBR), isoprene rubber or silicone rubber is used. These rubbers have been endowed with electrical conductivity by dispersing therein a conductive material such as carbon black or metal powder.
The resistance layer 23' is a layer for imparting an appropriate resistance to the charging roller 2', where polyamide resin, epichlorohydrin rubber, urethane rubber or silicone rubber is used. These materials have been endowed with an appropriate electrical conductivity by dispersing therein a conductive material such as carbon black or metal powder.
The protective layer 24' is provided in order to insure adequate surface performance of the charging roller 2' and in order to prevent the photosensitive member surface from being contaminated with the materials of the resistance layer 23'. Materials therefor may include polyamide resins such as N-methoxymethylated nylon, and urethane resins.
However, even when the photosensitive member surface is electrostatically charged by such contact charging, no uniform charging may be effected to cause faulty images due to uneven charging in some cases. This is because a developer (toner) adhering to the photosensitive member surface comes to adhere to the charging member though it is in a very small quantity, to cause uneven charging.
To solve such a problem, a method has been proposed in which the charging member is incorporated with fluorine resin particles in its surface portion (Japanese Patent Application Laid-Open No. 3-293682).
The above method, however, may cause a poor cleaning performance when the fluorine resin particles present at the surface of the charging member are mixed in a small quantity, especially in an environment of low temperature and low humidity, and hence may make the developer adhere to the charging member in a large quantity to cause uneven charging. Also, when the fluorine resin particles are mixed in a large quantity, it may be difficult to disperse the particles, so that film-forming properties may lower to cause cracks and peeling.
Japanese Patent Application Laid-Open No. 10-148995 discloses the concept of a method of incorporating fluorine resin in the protective layer. However, such fluorine resin not in the form of particles has a small effect on preventing uneven charging compared with that in the form of particles. Also, when it is incorporated in a large quantity so as to be effective, the protective layer may have a high releasability to have a low adhesion between it and its underlying layer. Especially where moisture condensation has occurred on the surfaces of the charging member and photosensitive member and their moisture-condensed surfaces have come into contact with each other, the water is confined at the contact nip between the charging member and the photosensitive member. This tends to cause a weld between the protective layer and the photosensitive member to cause "peeling" where only the protective layer of the charging member comes off partly, to remain on the surface of the photosensitive member.